* CEO Luetkestratkoetter resigns, replaced by Stieler
* Hochtief, Australian unit Leighton warn on 2011 profit
* Leighton announces rights issue to bolster balance sheet
* Hochtief shares down 9.1 percent, ACS down 2 percent
(Adds analyst, updates shares, adds links)
By Sonali Paul and Harro ten Wolde
FRANKFURT/MELBOURNE, April 11 (Reuters) - German bid-target Hochtief replaced its chief executive after a profit warning at its key Australian unit, taking some of the shine off Spanish predator ACS's attempts to gain control.
Frank Stieler -- who has been at the German contractor since March 2009 -- will take over from Chief Executive Herbert Luetkestratkoetter next month, Hochtief said.
One of the first tasks facing 52-year-old Stieler will be to fix problems at its Australian unit Leighton, which on Monday issued its third profit warning in five months and forced Hochtief to lower its full-year outlook.
Since late last year Spanish construction group ACS has been building a stake in Hochtief, banking on its German rival to help it diversify away from Spain's struggling building industry. Leighton was seen as the main prize of an eventual takeover.
Shares in Hochtief fell more than 9 percent, putting pressure on ACS, which last said it owned about 41 percent of the German contractor. ACS shares were down 2 percent.
"ACS has probably bought over half of what it needs to raise its stake in Hochtief to 50 percent, so the lower price at which it can buy the rest doesn't offset the fall in value of what it already has bought," a Madrid-based dealer said.
German brokers said last week they expected ACS to use any profit warning to pick up more shares and try to lift its stake above 50 percent.
Shares in Hochtief were down 9.1 percent by 1326 GMT, underperforming Frankfurt's 1.0 percent weaker mid-cap index and the wider STOXX 600 European Construction and Materials index, down 1.4 percent.
Luetkestratkoetter, who recently told a newspaper he would be comfortable staying on despite past differences with predator ACS, will step down on May 12 .
ACS said it backed Hochtief's incoming CEO.
Leighton, Australia's biggest contractor warned it would post a A$427 million ($450 million) full-year loss on mounting project costs and writedowns on its Middle East business. Hochtief expects its pretax profit to halve as a result.
Analysts had forecast a profit of A$426 million at Leighton for the year to June 30 ahead of the latest profit warning, according to Thomson Reuters I/B/E/S.
Leighton will also raise A$757 million to shore up its balance sheet, and Hochtief will spend about 300 million euros ($432.1 million) by taking up its full 54 percent entitlement.
Equinet analyst Ingbert Faust downgraded his rating for Hochtief from "hold" to "reduce", cutting his share price target to 60 euros from 87 euros.
"Uncertainty as regards further loss-generating projects in Leighton's order book increases (through this profit warning) markedly," Faust wrote in a note.
FATAL SITUATION
ACS's stake in Hochtief surpassed the 30 percent threshold in February, allowing the Spanish builder to raise its holding further without another mandatory bid.
Hochtief's shares have fallen nearly 10 percent in the two days between Leighton warning it was about to cut its guidance and announcing its new forecast. Leighton shares were on a trading halt on Monday.
"Luetkestratkoetter's resignation was not anticipated and the volume of the outlook corrections at Leighton are shockingly big," a Dusseldorf-based analyst said.
Leighton executives said the company would further boost its finances by selling some assets, without being more specific.
Some investors questioned the company's confidence that the worst was behind it.
"We're trying to work out if this is enough capital. Have they taken enough in the way of writedowns? You're taking all this on trust -- this is their third warning," said Richard Colquhoun, portfolio manager at Aviva Investors, which has an underweight position in Leighton shares.
Leighton's 1-for-9 entitlement offer, fully underwritten by sole lead manager UBS, is being priced at A$22.50 a share, a hefty 22 percent discount to its close last Friday. ($1 = 0.947 Australian Dollars) ($1=.6943 Euro) (Additional reporting by Matthias Inverardi and Josie Cox; Editing by Vinu Pilakkott, Louise Heavens and Erica Billingham)